This invention relates to an improved method of decontaminating surfaces contaminated with radioactive substances using electrolytic techniques.
The state of the art of electrolytically decontaminating radioactively contaminated surfaces is represented by U.S. Pat. No. 4,193,853. Therein is disclosed the electrolytic decontamination of such surfaces using a basic aqueous solution comprising nitrate, fluoride, borate and oxalate ions. The range of nitrate content is an amount equivalent to 50-500 g/l of sodium nitrate. The borate ion is included as tetraborate or pyroborate ion for two reasons. One is to provide the necessary basic pH, and the other is to function as a poison for unwanted neutron chain reactions or criticality excursions due to the presence in the solution of fissible materials such a thorium, uranium or plutonium. Oxalate is included primarily because of its ability to significantly increase the filterability of the precipitate which results upon electrolysis. It is this precipitate which contains the removed contaminants. Fluoride ion was included in this prior art electrolyte because it was believed that it improved decontamination efficiency.
However, it has now been found that this prior art electrolyte and the corresponding decontamination electrolysis method are significantly less efficient than desired. Although there are many disclosures relating to other electrolytes containing similar ingredients, none of these have been used in environments with the significantly different and stringent demands attendant to the decontamination of radioactive surfaces. Hence, U.S. Pat. No. 4,193,853 remains as the closest state of the art. Other disclosures of related electrolytes include those of U.S. Pat. No. 1,314,842 which employs nitrates of various metals in an acidic electrolitic etching solution which can also comprise oxalate and other anions such as barium; U.S. Pat. No. 1,314,839 relating to an acidic electrolytic etching solution containing borate anions of various metals and also, optionally, other salts including nitrates, oxalates, etc; U.S. Pat. No. 1,273,432 disclosing an acidic electrolytic etching solution which can also contain oxalate, tartrate, and other ions; and U.S. Pat. Nos. 3,106,199 and 3,228,816 disclose acidic electrolytes for cleaning and polishing aluminum and comprising nitric acid and ions such as fluoride, borate, etc. However, none of these relate to the decontamination of radioactive materials and use low concentrations of nitrate anions.